Invasive Species Management

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Need Noxious Weed Advice?

We are not able to provide site visits in 2021 but are monitoring calls and questions. See our contact information on the right-hand side of this page.

What We Do

Orange hawkweed flowerhead.The Jefferson County Invasive Species Management program provides the public with information and offers solutions for managing invasive species and supports good stewardship of the land.  

We ensure the compliance with Colorado’s noxious weed, forest pest, and agricultural vertebrate pest regulations on private, county, and state lands.  We also provide technical assistance and support to county departments who are responsible for managing county owned lands.

The program coordinates with private, local, state, and federal agencies to achieve regional pest control.

Who We Are

Canada thistle flower.Our staff includes the Invasive Species Management Coordinator and limited summer help.

Weed of the Month

Each month we will feature a noxious weed to help landowners identify weeds they may encounter on their property.

Diffuse knapweed

November 2021

Diffuse Knapweed

Centaurea diffusa

Centaurea x psammogena

Diffuse knapweed is one of the most prevalent noxious weeds in western North America.  In some places it can be a short-lived perennial but acts as a biennial throughout most of Colorado.

Diffuse knapweed has white to lavender flowers with tan bracts below.  The bracts are fringed on the sides and have a terminal spine.  They will sometimes have darker spots at the base of the spine but not always.

Diffuse knapweed vegetation contains alkaloids that may act allelopathically to prevent other plants from germinating.

The cells at the base of the plant undergo abscission in the fall which allows the plants to break off and tumble across the landscape, dropping its many seeds as it goes.  

It is difficult to distinguish diffuse from the hybrid in the field.  In fact, the majority is thought to be the hybrid.  The only way to definitively tell whether what you have is diffuse or the hybrid is through DNA analysis, which is expensive and not practical for most weed management programs. 

Control is easy if you can access the site at the best growth stage.  The optimum stage for removal or herbicide treatment is at the rosette to pre-bud stage.  There are also several biological control agents.  Check with the Insectary to see if they will work on your site.


Blair, A.C. and Hufbauer, R.A. (2010), Hybridization and invasion: one of North America’s most devastating invasive plants shows evidence for a history of interspecific hybridization. Evolutionary Applications, 3: 40-51

Know Your Knapweeds - NDSU

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A Landowner's Guide - Developing a Noxious Weed Management Plan

We developed this guide to assist landowners.  The information provides steps to develop a management plan for properties that have noxious weeds.

Download your copy

yellow_flag_iris Weed Spotter

The Weed Spotter program is a citizen science based effort to detect 6 high priority species on Jeffco Open Space parks.  Learn more at our Citizen Science page.